But it stopped short of saying that China's extensive censorship and surveillance apparatus currently conflict with its principles, or that it would refuse to offer any service there until conditions changed.

"Facebook has committed to freedom of expression and privacy standards... and we are independently assessed on our compliance on a biennial basis," the company said.

"In keeping with these commitments, rigorous human rights due diligence and careful consideration of free expression and privacy implications would constitute important components of any decision on entering China."

But, it added, "no decisions have been made around the conditions under which any possible future service might be offered in China."

Facebook has been blocked in China since 2009, when separatists from the country's mostly-Muslim Uighur minority used it to coordinate protests that escalated into riots. It continues to base staff there who sell ads to Chinese companies who wish to reach Western consumers.

Testifying before the committee in September, Facebook's chief operating officer Sheryl Sandberg said that it would "only operate in a country where we can do so in keeping with our values" and that this "will apply to China as well".

In its letter, the company said it did not know how China would seek to apply its laws against "harmful" content or content deemed to expose "state secrets" to Facebook even if it were allowed back into the country.

It also cited its membership of the Global Network Initiative (GNI), a digital rights organisation which commits its members to upholding human rights principles which would be difficult to square with China's policies.

Yet Google, which is also a member of GNI, has been widely criticised after being forced to acknowledge that it is considering a Chinese version of its search engine which would reportedly make it easier for the government to spy on its users.

Facebook's letter similarly said that the company would "fight to make sure that the service remains available for as many people as possible" in countries whose governments pass laws that conflict with its principles.